[q]Either Squirtle or Totodile. Pure water types that can learn ice type moves to get help them defeat their weakness to ground type pokemon. Mudkip's full evo (Swampert) is Water/Ground, so it becomes 4x weak to grass and probably won't get any ice moves off, and Piplup's full evo (Empoleon) is Water/Steel, and since steel is weak to fire it loses that advantage.[/q]

*apologies for failure to quote. Didn't notice the button, and assumed it was done via BBcode.*

You mean grass type (bold) I assume?
In any case, I've always thought Mudkip/Swampert were awesome because grass types typically aren't that common, are they? Or at least I personally hate them, except for Chikorita/Treecko. I figured electric types would be more common for some reason.

I also think that fire neutrality isn't much of a weakness for getting rid of grass-types. After all, I probably wouldn't want my Typhlosion going up against an Empoleon. The fighting type weakness is notable, though, so the steel dual-type does probably hurt more than it helps. (runs away from Blaziken)

^The above is very much based on opinion. I know absolutely nothing about competitive play, so take that with a healthy dosing of salt.